1. Field of the Invention
The present invention relates to a carborane supercluster and a method of producing the same. More particularly, the invention relates to a carborane supercluster containing boron-rich carborane (if carborane is given as the molecular formula C2B10H12, the boron content is approximately 75% by weight) with high thermodynamic and chemical stability as its unit constituent (i.e., a cluster molecule), and a method of producing the same.
2. Description of the Related Art
Carborane is generally a compound consisting of boron hydride into which carbon atoms are introduced which has a hollow-cage-shaped structure. Boron hydride (i.e., hydride of boron) is usually termed xe2x80x9cboranexe2x80x9d, because of its similarity to xe2x80x9calkanexe2x80x9d (i.e., saturated hydrocarbons).
A well-known borane is decaborane (14) expressed by a chemical formula B10H14, which is comparatively stable and industrially important. Decaborane (14) has a skeletal structure of boron (B) atoms, as shown in FIG. 1(a). The skeletal boron structure of FIG. 1(a), which is cage-shaped, constitutes a regular icosahedron formed by 12 boron atoms located at its respective vertexes and at the same time, the two adjacent boron atoms are removed therefrom. The molecular structure of decaborane (14) is that the remaining ten boron atoms of the skeletal structure are respectively terminated by hydrogen (H) atoms, where the two adjacent boron atoms including opened sites are bridged by hydrogen atoms at four positions.
o-Carborane (i.e., ortho-carborane, 1,2-dicarba-closo-dodecaborane), which is expressed by a chemical formula C2B10H12, is known as one of typical carboranes. o-Carborane has a similar molecular structure to the above-described molecular structure of decaborane (14), as shown in FIG. 1(b). Specifically, the four bridging hydrogen atoms are removed from the above-described molecular structure of decaborane (14) and then, two carbon atoms are respectively inserted to the two opened sites thereof, thereby completing the regular icosahedron. Moreover, the two carbon atoms and the ten boron atoms, which are respectively located at the 12 vertexes of the icosahedron, are respectively terminated by hydrogen atoms. Thus, the molecular structure of o-carborane is formed.
Actually, o-carborane is synthesized from decaborane (14). Specifically, if decaborane (14) is reacted with acetylene under the existence of a Lewis base such as acetonitrile and alkylamines, o-carborane is produced. o-Carborane has two isomers, m-carborane (i.e., meta-carborane, 1,7-dicarba-closo-dodecaborane) and p-carborane (i.e., para-carborane, 1,12-dicarba-closo-dodecaborane), according to the relative position of the two carbon atoms. m-Carborane has a molecular structure shown in FIG. 1(c), where one boron atom is located between the two carbon atoms. p-Carborane has a molecular structure shown in FIG. 1(d), where the two carbon atoms are located symmetrically with respect to the center of the regular icosahedron.
If o-carborane is heated at 425xc2x0 C. in an inert atmosphere, it is irreversibly isomerized to m-carborane. If o-carborane is heated at 700xc2x0 C. in an inert atmosphere, it is irreversibly isomerized to 75% of m-carborane and 25% of p-carborane.
In general, a geometrically-closed, cage-shaped borane cluster (which is expressed by a chemical formula BnHn, where n is a positive integer) has (n+1) bonding molecular orbitals in its cage-shaped skeletal structure. Thus, to close electronically the shell of the borane cluster, 2(n+1) electrons are necessary to fill these orbitals.
Regarding boron, a boron atom has three valence electrons, where one of the valence electrons is used for forming the Bxe2x80x94H bond. Therefore, the remaining two electrons are available to the formation of the skeletal structure. For example, with dodecaborane expressed as B12H12 (n=12), which is neutral and which has a skeletal structure of a regular icosahedron, 26 (=2(12+1)) electrons are necessary to fill the molecular orbitals to thereby close electronically the shell. Since dodecaborane contains 12 boron atoms 24 (=2xc3x9712) electrons are available to the formation of the skeletal structure. This means that two electrons are deficient. Accordingly, the shell of dodecaborane (B12H12) is unable to be closed electronically and thus, dodecaborane is instable.
Unlike this, with carborane (C2B10H12), where the two boron atoms of dodecaborane (B12H12) are respectively replaced with two carbon atoms, each of these carbon atoms provides three electrons available to the formation of the skeletal structure. This is because a carbon atom has four valence electrons while one valence electron is used for forming the Cxe2x80x94H bond. Therefore, carborane contains 26 (=2xc3x9710+3xc3x972) electrons available to the formation of the skeletal structure. This means that the bonding orbitals of carborane are filled and thus, its shell is closed electronically. Moreover, the molecular structure of carborane is geometrically closed. Due to the synergism of the geometrically-closed molecular structure and the electronically-closed shell, carborane is expected very stable.
In the following explanation of this specification, the word xe2x80x9ccarboranexe2x80x9d means a specific boron hydride expressed by C2B10H12 for the sake of simplification.
Boranes, which are used as the starting source material for synthesizing a desired material, are generally instable. In contrast, the carborane (C2B10H12) is chemically stable against reagents such as oxidizing agents, strong acids (heated, concentrated sulfuric acid and nitric acid), and alcohols. Moreover, the carborane (C2B10H12) is not deteriorated at high temperatures, which is simply isomerized even at 700xc2x0 C. as described previously. This means that the carborane (C2B10H12) is very stable not only chemically but also thermally.
To utilize the chemical and thermal stability of the carborane, conventionally, the carborane has been introduced into a macromolecule as a function group to thereby improve the characteristic of the macromolecule. These carborane-introduced macromolecules have been used for heat- and chemical-resistant piping materials, gaskets, membranes, and covering materials.
Furthermore, to make use of the excellent insulation properties of the carborane-introduced macromolecules, insulating/insulated gloves and insulating/insulated clothes have been developed and at the same time, various trials to use the macromolecules as the integrated-circuit insulator have been made. To utilize the rigidity or inflexibility of the carborane molecules, application as constituent elements of liquid crystals has been studied as well.
Moreover, since the boron content of the carborane is approximately 75 wt %, applications that use the properties of boron (which is a trivalent element containing three valence electrons) have been developed. Boron is usually used as a dopant for introducing holes into silicon (which is a tetravalent element containing four valence electrons). Similar to this, to produce a p-type semiconductor, there has been a trial to deposit the carborane on a silicon substrate by a CVD (Chemical Vapor Deposition) process.
Boron has two stable isotopes, 10B (19.8%) and 11B (80.2%). The isotope 10B has a very large cross-section (3.840xc3x9710xe2x88x92∞m2) with respect to thermal neutron capture and therefore, it has been used as a neutron capture agent. High-energy particles generated through the nuclear reaction 10B+n=4He+7Li+2.79 MeV (n: neutron, MeV: 106 electron volts) have a property that breaks all substances existing in their flying range. An application of this property to medical care is xe2x80x9cboron-neutron capture therapyxe2x80x9d, where a derivative of the carborane is administrated to a sufferer from a tumor to thereby enter an invaded organ and then, a neutron beam is irradiated to the tumor cells to necrotize the same.
Additionally, to utilize the nature of a complex of the carborane to capture metal ions, a research to collect radioactive ions from nuclear wastes to condense the same has been made.
Additionally, to utilize the nature of a complex of the carborane to capture metal ions, a research to collect radioactive ions from nuclear wastes to condense the same has been being made.
As described above, the carborane (C2B10H12) is available for heat- or fire-resistance materials, chemical-resistant materials, semiconductor materials, optical materials represented by liquid crystals, neutron-capturing agents in nuclear power applications, medicaments used for boron-neutron capture therapy (which utilizes a rays generated by the neutron capture property of boron), and agents for treating radioactive wastes. Therefore, it is highly useful or valuable in various industrial fields. However, there has been no substance or material where the molecules of the carborane are directly linked and accumulated.
It is known that if a substance is physically divided into pieces, the properties will differ remarkably from those extrapolated based on the properties of a bulk solid. In general, an ultrafine substance with an atomicity of approximately 103 or less is termed xe2x80x9cclusterxe2x80x9d. A cluster contains an extremely small number of constituent atoms and thus, the properties will vary conspicuously even if the count of the constituent atoms of a cluster is different from another by one. Moreover, since the bonding pattern or form between the atoms varies according to the kind of the atoms, the properties will differ greatly. This means that various properties can be developed or expressed by controlling the size of a cluster and/or choosing the constituent atoms.
Furthermore, if a microstructure is formed by a cluster or clusters with a specific size and a specific composition, a novel property or properties is/are expected to be born. In general, the minimum constituent unit of a cluster is an atom; however, a micro-substance consisting of clusters gathered with some chemical bonds is termed xe2x80x9csuperclusterxe2x80x9d, where the cluster is the minimum constituent unit. This is because this micro-substance consists of a xe2x80x9ccluster of the clustersxe2x80x9d. To generate such a supercluster, the cluster as the minimum constituent unit (i.e., the minimum unit cluster) needs to have and keep specific stability and specific uniqueness. Concretely, the first condition is that the minimum unit cluster is stable as one molecule. The second condition is that the minimum unit cluster keeps its properties at a certain level even after the minimum unit clusters are linked together to form a supercluster.
A supercluster is expected as a novel substance or material that expresses peculiar properties, similar to ordinary clusters. Thus, a technique of producing a supercluster stably and efficiently is extremely valuable in various industrial fields. However, it has been very rare to produce a supercluster actually except for C60 polymer (which is one of fullerenes) or the like. There has been the strong need to develop novel superclusters.
As described above, the carborane is a highly stable cluster with a geometrically- and electronically-closed shell. In other words, the carborane forms a cluster molecule. The properties of the carborane molecule are generally defined by the regular-icosahedron-shaped skeletal structure of the carborane itself. It is expected that the carborane molecule keeps its uniqueness even after it is used to form a supercluster, if the carborane molecules are linked together by dehydrogenating condensation (i.e., through hydrogen elimination of the carborane, which applies a small effect to the electron structure) while keeping the skeletal structure of the carborane unchanged.
In this way, although the carborane satisfies the conditions as the constituent unit of a supercluster, there has been no technique for realizing a carborane supercluster under the existing conditions. As a result, it is said that no supercluster consisting of the carborane has been developed so far.
Accordingly, an object of the present invention is to provide a carborane supercluster consisting of carborane (C2B10H12) as its constituent unit (i.e., a cluster or molecule) and a method of producing the same.
Another object of the present invention is to provide a carborane supercluster consisting of carborane (C2B10H12) as its constituent unit, the structure of which is controllable on an atomic-size (i.e., nanometer) scale, and a method of producing the same.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to the first aspect of the invention, a carborane supercluster is provided. This carborane supercluster consists essentially of linked clusters of carborane (C2B10H12), where the cluster serves as a constituent unit of the supercluster. This carborane supercluster provides novel substances that have never been reported so far, which is expressed as a molecular formula C2mB10mH12m-x, where x is a positive integer (i.e., x=1, 2, 3, . . . ) and m is an integer greater than unity (i.e., m=2, 3, . . . ).
The parameter x represents the count of removed or detached hydrogen atoms. The parameter m represents the count of the clusters linked together. For example, if m=2, the supercluster will be a dimer of the cluster. If m=4, the supercluster will be a tetramer of the cluster.
With the carborane supercluster according to the first aspect of the invention, the cluster or molecule of the highly thermodynamically and chemically stable carborane (C2B10H12) serves as a constituent unit of the supercluster of the invention. Therefore, the carborane supercluster of the invention is applicable to various industrial fields. For example, it is applicable to heat- or fire-resistant materials, chemical-resistant materials, semiconductor materials, neutron capturing materials in the nuclear power plants, medicaments for boron-neutron capture therapy, materials or agents for processing the nuclear waste, and optical components or materials, and their intermediates.
Moreover, the carborane supercluster of the invention is constituted by the cluster or molecule of the highly thermodynamically and chemically stable carborane (C2B10H12). Thus, the structure of the supercluster is controllable on an atomic-size (i.e., nanometer) scale. This means that the supercluster of the invention is applicable to ultrasmall, precision parts or components having specific atomic arrangements in a variety of future industrial fields such as chemical and electronic industries.
Preferably, the cluster of the carborane consists of o-carborane. Alternately, the cluster of the carborane consists of m-carborane or p-carborane. The cluster of the carborane may consist of at least two of o-carborane, m-carborane, and p-carborane.
Preferably, the count of the linked clusters is set at five or thirteen.
According to the second aspect of the invention, a method of producing a carborane supercluster is provided. In this method, carborane (C2B10H12) is ionized to generate carborane ions. Then, the carborane ions thus generated are successively reacted with neutral (i.e., non-ionized) carborane (C2B10H12), thereby generating the supercluster expressed by C2mB10mH12m-x having a cluster structure of the carborane (C2B10H12).
With the method of the second aspect, the supercluster according to the first aspect of the invention is produced.
According to the third aspect of the invention, another method of producing a carborane supercluster is provided. In this method, carborane (C2B10H12) ions are electrically confined in a specific region. Then, the carborane ions thus confined are successively reacted with neutral (i.e., non-ionized) carborane (C2B10H12), thereby generating the supercluster expressed by C2mB10mH12m-x having a cluster structure of the carborane (C2B10H12).
With the method of the third aspect, the supercluster according to the first aspect of the invention is produced.
Even if neutral (i.e., non-ionized) carborane (C2B10H12) is successively reacted with each other in solid or liquid phase, the supercluster expressed by C2mB10mH12m-x according to the first aspect of the invention is unable to be produced. To produce the supercluster of the first aspect, neutral (i.e., non-ionized) carborane (C2B10H12) needs to be successively reacted with the ionized carborane (i.e., carborane ions) in gas phase.